The present invention relates to strain sensors and methods for measuring strain and, more particularly, to the use of shape memory alloys for measuring large strains in objects and to devices containing shape memory alloys for monitoring strain in the devices.
Shape memory alloys (SMAs) are metal alloy materials that have the ability to return to their original shape after being deformed. All SMAs have two distinct crystal structures, or phases, with the phase present being dependent on the temperature and the amount of stress applied to the SMA. The two phases are martensite, which exists at lower temperatures, and austenite at higher temperatures. The exact structure of these two phases depends on the type of SMA, where the most commonly used type is called Nitinol. Nitinol is a mixture of two component metals, nickel (Ni) and titanium (Ti), which are mixed in an approximate ratio of 55% by weight Ni and 45% by weight Ti, and annealed to form a part in the desired shape.
Shape memory alloys possess two material properties that work together to provide shape memory. The first material property is an austenite to martensite transition in the SMA. This is a solid-to-solid phase transition from an austenite phase with high symmetry (such as a cubic molecular structure) to a martensite phase with lower symmetry (such as tetragonal or monoclinic structures). The second property of a shape memory alloy is the ability of the low-symmetry martensite structure to be deformed by twin boundary motion. A twin boundary is a plane of mirror symmetry in the material. If the twin boundary is mobile, as in certain martensite structures, the motion of the boundary can cause the crystal to rearrange and thus accommodate strain.
Pseudoelasticity (also known as superelasticity) uses the same deformation mechanisms as shape memory, but occurs without a change in temperature. Instead, the transformation is induced by stress alone. Applied stress can overcome the natural driving force which keeps the material at equilibrium in the austenite phase. By applying stress to the material, it can be converted into the martensite phase, and the crystal structure will strain to accommodate the applied stress. When this stress-energy is greater than the chemical driving force of stabilization in the austenite phase, the material will transform to the martensite phase and be subject to a large amount of strain. When the stress is removed, the material returns to its original shape in the austenite phase, since martensite cannot exist above the transition temperature. This superelastic behavior is fully reversible and does not require any change in temperature.
Electrical-type strain gauges are typically used for measuring strain. One common type is a resistance strain gauge, which measures an elongation of an object experiencing a mechanical load. The resistance of an electrically conductive strain gauge material is proportional to the elongation caused by an elastic deformation of the material. The measured change of resistance is converted to an absolute voltage by a wheatstone bridge circuit, and the resulting voltage is linearly related to strain by a constant known as a gauge factor.
A strain sensor/gauge made of a shape memory alloy material, preferably a pseudoelastic alloy material, and a method for measuring strain is disclosed. A preferred pseudoelastic alloy is Nitinol, which exhibits a measurable change of resistance when strained. A strain gauge can be constructed with an element made of the pseudoelastic alloy mounted on a substrate, which is capable of elongating to accommodate the elongation of the pseudoelastic alloy. Preferably a strain gauge comprising a Nitinol element is mounted on a substrate, which is mounted on an object to measure strain in the object. Preferred substrates include high temperature, high performance thermoplastics such as PEEK, PEI, and PPS; and lower temperature, lower melt viscosity thermoplastics like Grilamid and Kraton materials.
In another preferred form of a strain gauge according to the present invention, the strain gauge comprises an element made of a pseudoelastic alloy which can be stitched or woven into a web of material (such as a fabric) for measuring strain in the web. In such a strain gauge, the pseudoelastic alloy can strain up to approximately 8% of its length without permanent deformation. When stitched to a fabric, the strain gauge element can measure strains of up to approximately 8% in the fabric. When an element (e.g. a filament) of pseudoelastic material is woven into fabric, the strain gauge comprising that filament can measure strains of up to approximately 30% in the fabric.
The method and article of the present invention is particularly useful for measuring strains in webs of material subject to large applied stresses, in which strain gauges often deform by greater than approximately 2% elongation. Strain gauges according to the present invention can be used in applications such as: parachute static lines, parachute canopy materials, and automotive and aircraft seatbelts. When a strain gauge element is woven into a web in one of the above applications, the element can elongate by up to approximately 8% and measure elongations in the web of up to approximately 30%, with any elongation beyond approximately 20% generally not being recoverable by the web. Conventional strain gauges made of typical metals and metal alloys fail when the metal material(s) reach approximately 0.1-1% elongation. Thus, it is not possible to measure moderate to high strain amounts using these typical materials. It has now been discovered that strain in materials that experience straining or stretching by greater than about 1%, and more preferably greater than about 2%, in response to applied stresses can be monitored using strain gauges comprising a pseudoelastic material that exhibits recoverable strain greater than about 1%, and preferably greater than about 2%.
A strain gauge including an element, such as a filament or wire, made of a shape memory alloy and/or a pseudoelastic alloy material such as Nitinol exhibits a change of resistance when it is strained, similar to conventional strain gauges. Thus, conventional strain gauge signal conditioning techniques can be used to measure strain in accordance with the device and method of the present invention.
As used herein, the terms xe2x80x9cshape memory alloyxe2x80x9d and xe2x80x9cpseudoelastic alloyxe2x80x9d refer to a material having (i) an austenite to martensite solid-to-solid phase transition, and (ii) an ability for the martensite structure to be deformed by twin boundary motion. The preferred materials to be used in the present invention are pseudoelastic alloys, which are further defined as materials that undergo the martensite to austenite phase transition without a significant change in temperature. In pseudoelastic alloys, the martensite to austenite transition occurs due to the dynamically applied stress forces which overcome the natural driving force that keeps the material at equilibrium in the austenite phase.